The present invention relates to a stopping motor control apparatus for controlling the rotational angle of a rotor of a stepping motor.
Stepping motors find extensive applications in varieties of business machines such as printers, facsimiles, copying machines and the like. The stopping motor is well controlled, allowing a single pulse to control the rotation of a rotor thereof by a predetermined angle of rotation for carrying objects in a accurate manner highly. In the following, the operation of such a stepping motor will be described taking as an example a printer which utilizes the stepping motor.
A printer of this type comprises a print head mounted on a carriage, in which the print head includes a plurality of printing wires placed thereon in a direction of a printing sheet being carried. The carriage is spaced perpendicularly to the direction of the printing sheet being carried, and simultaneously the printing wires are selectively protruded to impact against the printing sheet via an inked ribbon. The printer thereby prints character information in the form of a dot matrix and a dotted image, etc., along a line thereof, and repeats the operation over a plurality of lines by feeding the printing sheet repeatedly.
The printer further includes a platen around which a printing sheet is wound. To feed the printing sheet in synchronism with the printing operation, i.e., to effect the so-called line-feed, a line feed motor composed of a pulse motor is provided. A stepping is supplied to the line feed motor for rotating the platen by an angle of rotation corresponding to the amount of the line feed of the printing sheet in a predetermined timing established from a host side. The line feed motor hereby drives the platen to feed the printing sheet by an instructed distance.
After the line feed, the line feed motor is allowed to have predetermined retaining torque. The retaining torque is set so as to be enough to prevent a printing sheet from being shifted with ease by an external force and further to permit an operator to feed the printing sheet manually. In fact, the retaining torque ranges from about 1/5 to 1/10 as large as the torque of the line feed motor when it feeds the printing sheet.
Referring here to FIG. 1, a block diagram of such a printer controller is illustrated. The controller includes a printing part 9 connected to a microprocessor 2, MSM80C154 for example, available from Oki Electric Industry Co., Ltd., via a bus line 1. The microprocessor 2 effects arithmetic operation for the control of the printer. The printing part 9 is comprised of a print head for printing any character on a printing sheet under the control of the microprocessor 2, and of various mechanisms associated with the printer. The control circuit is further comprised of a read only memory (ROM) 3, such as an Intel D27512-NW for example, for storing operating programs, etc., for use in the microprocessor 2, and a random access memory (RAM) 4, such as a Hitach HM62256LP-15 for example, for storing various data required for the operation of the microprocessor 2 such as the arithmetic operation, both memories being connected to the microprocessor 2 via the bus line 1 of a driver circuit 5 connected to the bus line 1 and to an line feed (LF) motor 6, for issuing a drive pulse to the LF motor 6, and of an interface (I/F) control part 7, NEC .mu.PD8255AC-2 for example, connected to the bus line 1 for receiving serial data input from a host side via an I/F connector 8 connected thereto and for oppositely transmitting predetermined information to the host side via the I/F connector 8.
Referring further to FIG. 2, the driven circuit 5 of FIG. 1 is illustrated in detail. The LF motor 6 shown is a unipolar driving two-phase excitation stepping motor wherein a rotor 13 is rotated by rotating a composite magnetic field vector produced by a pair of excitation coils 11, 12 wound around a stator. The well-known drive circuit 5 receives at input terminals 21 to 24 rotation control signals S1 to S4, shifted .pi./2 in their phases, and at an input terminal 20 a driving control signal S0. The control signals S0 to S4 are fed from the microprocessor 2 of FIG. 1. The input terminals 20 to 24 are connected to the bus line 1 of FIG. 1. The rotation control signals S1 to S4 are fed to NPN transistors T1 to T4 which send in turn excitation currents to the excitation coils 11, 12 through drivers K1 to K4. Additionally, the driving control signal S0 is fed to a PNP transistor T5, which constitutes a switching circuit together with resistors R5 and R6, through a driven K0 and a NPN transistor T0. The switching circuit controls, when it is on, the excitation of the excitation coils 11, 12 in a predetermined timing for rotation of the rotor 13. The drivers K0 to K4 are pulled up through resistors R0 to R4 by voltage supplied from a power supply (not shown) to a terminals 25. The excitation currents for the excitation coils 11, 12 are supplied, when the transistor T5 is switched on, from a power supply (not shown) through a terminal 26. A terminal 27 supplies a retaining current, when the transistor T5 is switched off from a power supply (not shown) through a resistor R7 and a diode D0 to the excitation coils 11, 12 when the rotor 13 is stopped and any two of the transistors T1 to T4 are turned on, for retaining the rotor 13 with predetermined retaining torque. The driving control signal S0 is intermittently supplied at a predetermined repetition frequency during the rotation of the LF motor 6. The driving control signal S0 controls the driving of the LF motor 6, with which signal the LF motor 6 is driven in case the signal S0 is substantially turned on without interruption. Application of each pulse of the driving control signal S0 rotates the rotor 13 by an electrical angle of .pi./2.
Referring to FIG. 3, a speed reduction mechanism of the LF motor is illustrated. A spur gear 31 mounted on an output shaft of the LF motor 6 is engaged with a large diameter spur gear 32, which is fixedly mounted on a small diameter spur gear 33 coaxially therewith. The small diameter spur gear 33 is further engaged with a large diameter spur gear 34, which is coupled directly to a platen shaft 35. The spur gear 31, spur gears 32 and 33, and spur gear 34 are hereinafter referred to as a motor gear, idle gears, and a platen gear.
Referring further to FIGS. 4(a) to 4(b), printing wires of the print head provided in the printing part 9 of FIG. 2 are schematically illustrated. The print head is composed of a disk-shaped base, a cylindrical permanent magnet, a plurality of electromagnets located in close vicinity to the permanent magnet on the internal periphery side thereof, and armatures each provided on divided pieces which are formed by dividing a circular leaf spring coaxially, for fixedly mounting thereon the printing wires. Upon printing, respective electromagnets are selectively evergyzed, and the armatures attracted by the respective electromagnets to cores thereof are released by a biassing force of the leaf spring for the printing by the printing wires.
As illustrated in FIGS. 4(a) to 4(b), (a) shows printing wires, on the heads thereof, of the 9 wire type head, (b) and (c) show printing wires of the 18 wire type heads each arranged in 2 lines .times.9 wires, and (d) shows printing wires of the 24 wire type head arranged in 2 lines .times.12 wires. In FIGS. 4(b) to (d), only three wires are shown for each line of the wires. A printing sheet is fed along the surface of the drawing vertically on the same. The nine printing wires in FIG. 4(a) are spaced apart 1/72 inch (1 inch=2.54 cm) for example. Such a dot wire type print head typically allows overlapped printing where a distance between adjacent longitudinal printing dots is shortened as shown by a broken circle in the figure. It is therefore necessary to rotate the platen by a multiple of an integer of 1/2 as much as the distance between adjacent printing wires. The platen should accordingly be driven each 1/144 inch. The same shall be applied to the 18 wire type head shown in FIG. 4(b). Here, two lines of the wires of the 18 wire type head of FIG. 4(b) are shifted with each other vertically on the drawing surface by 1/2 of the wire diameter, and the wires of FIG. 4(c) are located vertically at the same positions for each line. Additionally, in the 24 wire type head of FIG. 4(d) the adjacent printing wires are spaced apart 1/90 inch, requiring the platen to be driven in the step of 1/180 inch. Likewise, the 24 wire type head is required to be driven in the step of 1/120 inch.
Conventionally, a stepping motor (LF motor) 6 is rotated 7.5.degree. for each step of rotation.
Operation of the control circuit of the printer arranged as such is as follows.
As illustrated in FIG. 1, printing data is input from the host side into the interface control part 7 via the interface connector 8, and stored in the RAM 4. The microprocessor 2 reads the printing data and prepares a predetermined printing signal for driving the printing part 9. The microprocessor 2, after completing the printing of one line, supplies a line feed signal to the drive circuit 5. That is, the microprocessor 2 supplies the driving control signal S0 and the rotation control signals S1 to S4 shown in FIG. 2 to the drive circuit 5 of the LF motor 6 for a predetermined time interval. The LF motor 6 is thereby rotated by a predetermined angle of the rotation.
With the LF motor 6 being driven as such, the rotary speed thereof is reduced in a predetermined ratio through the motor gear 31, idle gears 32, 33, and platen gear 34, for rotation of the platen by a predetermined angle.
Referring here to FIG. 5, a relationship between the torque of the LF motor and the number of revolutions thereof (rotary speed) is illustrated. With the rotary speed of the LF motor taken on the horizontal axis, the driving torque of the LF motor is steeply lowered as the rotary speed exceeds 90% assuming the maximum rotary speed to be 100% at which the LF motor can follow up a driving pulse. Thus, the LF motor may often be operated at the rotary speed of about 70%.
Here, there are many types of printers capable of mounting varieties of the print heads as shown in FIG. 4. It is desirable thereupon to employ the same driving mechanism for platens of those printers for the purpose of reducing the number of parts needed.
For example, printers mounting thereon the 9 wire type print head of FIG. 4(a) and the 24 wire type print head of FIG. 4(d) employ respectively different, for example, gear ratios (ratio of the number of cogs) with respect to the idle gears (spur gears 32, 33 of FIG. 3) and the platen gear (spur gear 34 of the same figure). The LF motor 6 is also commonly used for those different situations. There are required, however, different parts such as platen gears and idle gears for each type of the printers, thus insufficiently reducing the number of parts needed. Additionally, alteration of the speed reduction ratio (gear ratio) causes an increase or a decrease of the torque of the LF motor as illustrated in FIG. 5. That is, the decrease of the motor torque disadvantageously produces insufficient force of feeding a printing sheet by a platen. Conversely, the increase of the motor torque produces an increase of the retaining torque, making operation difficult when setting a printing sheet by manual rotation of the platen by an operator.